Regulation of protein A synthesis by the sar and agr loci of Staphylococcus aureus

The synthesis of protein A in Staphylococcus aureus is regulated by global regulatory loci such as sar and agr. Phenotypic data indicate that both sar and agr suppress protein A synthesis; like agr, sar also regulates protein A production at the transcriptional level. To determine the genetic requirement of sar in protein A suppression, we transformed shuttle plasmids containing various sar fragments into a sar mutant. Our results indicated that the 560-bp sarA transcript, or, more probably, the SarA protein (13.5 kDa), is sufficient for suppressing protein A gene transcription when introduced on a multicopy plasmid or as a single copy in the chromosome. Immunoblot analysis with a chicken anti-protein A antibody also confirmed the reduction in protein A expression in these sar mutant clones. Complementation studies revealed that the transcription of the protein A gene can be suppressed in a sar mutant background by a plasmid containing RNAIII. Surprisingly, in agr deletion mutant clones and in clones derived from the agr-sar double mutant, protein A gene transcription can also be suppressed by plasmids containing the sarA transcript plus additional upstream sequence but not the sarA transcript alone. These data suggest that the sar locus can down-modulate protein A gene transcription via both RNAIII-dependent and RNAIII-independent pathways. Consistent with the hypothesis of an RNAIII-independent pathway is an additional genetic requirement for protein A suppression in the agr deletion mutant RN6911 as well as the isogenic double sar-agr mutant, whereas in the sar mutant background, the sarA transcript encoding the SarA protein alone is sufficient. These data suggested that both sar and agr are coregulators of protein A synthesis in S. aureus.